Design Hydraulic Study. Bridge 09C-0134, Blairsden-Graeagle Road over Middle Fork Feather River. Plumas County. Prepared for:

Transcription

1 Design Hydraulic Study Bridge 09C-0134, Blairsden-Graeagle Road over Middle Fork Feather River Plumas County Prepared for: Quincy Engineering, Inc 3247 Ramos Circle Sacramento, CA Prepared by: Pacific Hydrologic Incorporated 1062 Market Street Redding, CA Norman S. Braithwaite, P.E. December 3, 2013

2 TABLE OF CONTENTS Introduction 1 Description of Basin 2 Description of Stream and Site 2 Hydrologic Analysis 3 Hydraulic Analysis 5 Scour and Erosion 7 Other Considerations 8 Conclusions and Recommendations 9 TABLE 1, Stream and Gaged Basin Characteristics 4 TABLE 2, Estimated 50- and 100-Year Flood Peak Flows 4 TABLE 3, Existing Hydraulic Conditions 6 TABLE 4, Preferred Bridge Hydraulic Conditions 7 TABLE 5, Total Potential Scour 8 PHOTOS FIGURES APPENDIX A APPENDIX B APPENDIX C APPENDIX D Additional Hydrologic Data Additional Hydraulic Data Scour Calculations References ii

3 Executive Summary Purpose: Funding Program: Design Flood: Clearance for Drift: Design Exception: Replace deficient bridge. HBP Standard Design Flood (21100-cfs, 50-year recurrence) 3.0-feet None required for flood hydraulic conditions Recommendations: Min. Soffit Elevation feet (to meet recommendations of Caltrans and FHWA) Pier Scour Elevation feet Abutment Scour Elevation Abutment 1, 4339-feet Abutment 3, 4346-feet Abutment Protection Recommended to reduce the long term potential for damage to abutments from bank erosion and bank migration. Note regarding estimates of potential scour: Potential scour has been estimated using empirical equations presented in FHWA HEC-18. These equations do not consider geotechnical conditions and therefore assume all substrate is erodible. The potential scour estimates identified in this report may be inappropriate if a geotechnical investigation identifies material resistant to erosion at higher elevations. Preferred Bridge Characteristics: Soffit Elevation feet (4.59-ft above Q50, 3.13-ft above Q100) Overtopping Flood Impact on Flood Risk Impact on Channel cfs, approximately 150-year recurrence Minor (0.18-foot) increase in water surface elevation during the most probable 100-year flood but no increase in flood risk to structures because no structures are present within the 100-year floodplain. Preferred bridge is not expected to aggravate channel instability. iii

4 Design Hydraulic Study Blairsden-Graeagle Road over Middle Fork Feather River INTRODUCTION Background: This bridge hydraulic analysis has been prepared for the sole purpose of meeting the requirements of 23 CFR and dealing with bridges, structures, and hydraulics. Although potentially useful for other purposes, this analysis has not been prepared for any other purpose. Reuse of information contained in this report for purposes other than those for which this analysis and report are intended is not endorsed or encouraged by the author and is at the sole risk of the entity reusing information herein contained. Estimates of peak flows for frequent flood peaks (5-year or more frequent), if shown in this report, should not be considered accurate unless an overtopping flood of 5-year or more frequent is identified. Analyses to meet the requirements of FEMA, the State of California Reclamation Board, low flow environmental or construction concerns and for other purposes may be provided as additional services. Design Standards: Funding: Hydraulic design of the preferred bridge is based on standards recommended by Caltrans (Local Programs Manual - reference 1). Exceptions to these design standards are recommended only if meeting the standard is found to be impractical or unreasonably costly for the proposed project and the exception does not result in an increased risk of damage during floods. Local design standards provided in writing prior to the preparation of the hydraulic analysis have also been considered. HBP Existing Bridge: Year Constructed Truss 1910, Approaches 1965 Length feet nominal (261-ft effective) Clear Width 14.1-feet Total Width 18.0-feet Skew (hydraulic) None Lanes 1 Speed Limit Not posted Load Limit None Structure 6-span steel stringer, 1-span Pratt through truss, 2-spans steel stringer. The existing bridge will not be removed. Deficiency Function Sketch Figure 1, page 14 Photos 1-4, pages 11, 12 Significance: Description of Service Forest resources, alternate route for SR-89 Length of Detour 2-miles on roads of equal or greater service Description of Road Straight, flat 1

5 Preferred Bridge: Length feet nominal, feet effective hydraulic Clear Width 26.0-ft Total Width ft Skew (hydraulic) 10-degrees Lanes 2 Speed Limit Not posted Load Limit None Structure Two span CIP/PS Concrete Box Girder Traffic During Construction Maintained on existing bridge General Plan Figure 2, page 15 DESCRIPTION OF BASIN Geographic Location: Above Blairsden-Graeagle Road, the Middle Fork Feather River drains a small basin on the northern end of the Sierra Nevada Mountain Range. Receiving Waters: Characteristics: Land use: Vegetation: Geologic: Sacramento River Area of basin 711 sq-mi. Shape Tenticular rectangle Highest elevation 8800-ft Babbitt Peak on the southeast border of basin Lowest elevation 4350-ft near bridge Elevation index 5.1 Average annual precipitation (basin wide) 25-in Aspect North Forest resources. rural residential, small farms and ranches Conifer forest Topographic features indicate moderate potential for significant landslides capable of causing channel instability and risk to bridge integrity. Basin: Figure 3, page 16 DESCRIPTION OF STREAM AND SITE Stream Channel: In the vicinity of Blairsden-Graeagle Road, the Middle Fork Feather River is wide, shallow, and mildly meandering. The channel is confined along the right bank by a hillside. Along the left bank and on the inside of a large bend is a low, wide floodplain with a top elevation near that of the 100-year flood. The Middle Fork Feather River is shown in Photos 5 and 6 (page 13). 2

6 Stream Banks: Existing Bridge: The banks of the Middle Fork Feather River consist of alluvium on the left and colluvium on the right with a moderate cover of grasses, willow, and evergreen trees. The existing Blairsden-Graeagle Road bridge over Middle Fork Feather River is an eight span structure aligned perpendicular to the channel. Site Topography: Figure 4, page 17 HYDROLOGIC ANALYSIS Hydrologic Stability: Infrequent floods in Middle Fork Feather River are substantially natural and not significantly influenced by land use activities within the drainage basin. Flood History: Floods in Middle Fork Feather River have not been known to overtop Blairsden-Graeagle Road. Number of Methods: Two methods were investigated for estimating potential infrequent flood peak flows in Middle Fork Feather River. These include adjustment (translation) of known flood frequency curves at a proximate streamgage and direct application of the USGS Sierra Region Equations. Translation Analysis: Approach Translation analysis consists of estimating the infrequent flood peak flows by comparison with gaged stream or river basins. After identification of representative gaged basins, flood frequency relationships for the gaged basins are determined by plotting annual flood peaks and computing the normal probability Log-Pearson Type III curve fit (reference 7). If the Log-Pearson type III curve fit reasonably represents the plotted data for the less frequent floods, it is considered representative of the gaged basin and used as a basis of comparison. If not, a line of best visual fit may be used as a basis of comparison. After identifying representative flood-frequency relationships for the gaged basins, candidate flood frequency relationships representing the stream or river at the proposed project site are estimated by adjusting the gaged basin flood frequency relationship to account for differences in characteristics between the gaged basin and the basin above the proposed project. The adjustments are made using the area, elevation and precipitation exponents of the appropriate USGS region equation (reference 8). Basin Characteristics Characteristics of gaged basins found to be potentially representative of the basin above the proposed project and having records of adequate length to reasonably identify the infrequent flood peak flows are identified in Table 1. 3

7 TABLE 1 Stream and Gaged Basin Characteristics Basin Description Middle Fork Feather River at Blairsden-Graeagle Road Middle Fork Feather River near Clio USGS Gage Number Area (sq mi) Average Annual Precip (in) Elevation Index Years of Record n/a n/a Gaged basin flood frequency curves Plotted flood frequency data and curves for the gaged basins used in this analysis are shown in Appendix A. Regional Equations: Approach The USGS has published a set of regional equations for estimating infrequent flood peak flows in ungaged natural streams and rivers (not affected by lakes, reservoirs, substantial development or substantial reclamation projects) throughout most of California (reference 8). These equations are useful for planning level and rough preliminary estimates of infrequent flood peak flows and corroboration of flood frequency estimates using more detailed procedures. Flood peak flows estimated by these equations should only be relied upon for design if confidence in other methodologies is low and if verified by other methodologies. The empirical equations estimate flood peak flows from basin characteristics including area, elevation index and precipitation. Use of the area, elevation index and precipitation factor exponents of the regional equation for adjustment of flood characteristics from representative long term gaged basins (described in Regional Analysis above) is generally considered to provide a more reliable estimate of infrequent flood peak flows for the ungaged basin. Flood Peak Flows: Candidate flood frequency relationship All candidate flood frequency curves derived from regional analysis for the proposed project site are plotted and shown in Appendix A. Estimated 50- and 100-year flood peak flows from all methods investigated are summarized in Table 2. TABLE 2 Estimated 50- and 100-year Flood Peak Flows Estimated from 50-Year (cfs) 100-Year (cfs) Spanish Creek above Blackhawk Creek at Keddie USGS Sierra Region Equations Selected flood frequency relationship The flood frequency relationship estimated from Middle Fork Feather River near Clio has been selected as most appropriate for design of the replacement bridge. The estimate from the USGS Sierra Region Equations was not selected because these 4

8 equations are intended to provide a rough estimate of flood peak flows in the Sierra Nevada Mountains when local data is unavailable. The selected flood frequency relationship is shown in Figure 5 (page 18). Flood of Record: cfs on February 1, 1963 but a higher peak flow was likely to have been experienced January 1, 1997 after the streamgage was out of service. HYDRAULIC ANALYSIS Backwater Model: Backwater program The Corps of Engineers HEC-RAS version backwater program (reference 3) has been selected for modeling hydraulic characteristics representing existing conditions, preliminary bridge configurations and the preferred bridge. This program has been selected because of its long history of use (derived from HEC-2), wide acceptance and great flexibility for evaluating bridge configurations. Cross-section data Stream cross-sections and Manning s roughness coefficients upstream and downstream of the proposed project have been assumed constant for all models. Cross-sections used in the backwater models were from a recent ground survey. Locations of cross-sections used in the backwater model are shown on Figure 6 (page 19). Crosssections have been adjusted for skew as appropriate. Elevation Datum NAVD88 Manning s Roughness Coefficients Mannings Roughness Coefficients for the channel and banks were estimated by observation and comparison with similar channels identified in Roughness Coefficients of Natural Channels (reference 6). Manning's roughness coefficients ranging from to were used to represent the channel and ranging from to were used to represent the banks. Contraction and Expansion Coefficients Contraction and expansion coefficients of 0.1 and 0.3 respectively were used to represent the natural channel. These were raised to 0.3 and 0.5 respectively in the vicinity of the existing bridge. Downstream starting water surface elevation assumption The normal depth method in HEC-RAS was selected for estimating the downstream water surface elevation. A slope of 0.002, estimated from the slope of the stream channel, was used as the starting slope. Four surveyed crosssections and two interpolated cross-sections were used to isolate the effects of downstream starting water surface elevation assumption from water surface elevations at the bridge. 5

9 Existing Bridge: Purpose The existing condition backwater model has been prepared to identify and document existing hydraulic conditions and to serve as a basis of comparison with which to evaluate preliminary and preferred bridge configurations. Channel roughness coefficient at bridge to Bank roughness coefficient at bridge to Contraction coefficient 0.3 (at bridge) Expansion coefficient 0.5 (at bridge) Bridge modeling method Energy. Drift assumption 2 x actual pier width with 3-foot minimum Figure 7 (page 20) shows how existing bridge is represented in model. Model results Existing flood hydraulic conditions are summarized in Table 3. Existing condition flood profiles and a stage discharge curve at cross-section 2310 are shown in Figures 9 and 10 (pages 22, 23). Summary output tables from the existing condition HEC-RAS backwater model are included in Appendix B. TABLE 3: Existing Hydraulic Conditions upstream of existing bridge (with drift except as noted) Flood Flow (cfs) Recurrence (years) W.S. Elevation 1 (feet) Avg. Channel Velocity 2 (fps) Standard Design Base Base (no drift) Flood of Record < ± 9.5± Overtopping Flood ± ± Notes: 1) At cross-section 2310 located approximately 40-feet upstream of the existing bridge. 2) Highest average channel velocity in vicinity of bridge. Preliminary Bridges: Backwater models were prepared to represent three candidate bridge configurations. Results from these models were provided to project staff in the form of a preliminary hydraulic analysis report. Using information provided in the preliminary hydraulic analysis report and considering additional factors not related to hydraulic conditions, a bridge configuration was selected as the preferred bridge for final design. Preferred Bridge: The preferred bridge backwater model has been prepared to identify hydraulic requirements and impacts of the preferred bridge. Channel roughness coefficient at bridge to Overbank roughness coefficient at bridge to Contraction coefficient 0.3 (at bridge) Expansion coefficient 0.5 (at bridge) Bridge modeling method Energy. Drift assumption Pier assumed 11-feet wide (2 x actual pier width) 6

10 Figure 8 (page 21) shows how preferred bridge is represented in model. Model results Preferred bridge hydraulic conditions are summarized in Table 4. Preferred bridge flood profiles and a stage discharge curve at cross-section 2310 are shown in Figures 9 and 10 (pages 22, 23). Stage discharge curves at cross-sections 2250 for bridge design and 4080 for flood risk assessment are shown in Figures 11 and 12 respectively (pages 24, 25). Summary output tables from the preferred bridge HEC-RAS backwater model are included in Appendix B. TABLE 4: Preferred Bridge Hydraulic Conditions (with drift except as noted) Flood Flow (cfs) Recurrence (years) W.S. Elevation 1 (feet) Avg. Channel Velocity 2 (fps) Standard Design Base Base (no drift, x-sec 2310) Flood of Record < ± 9.0± Overtopping Flood ± ± Notes: 1) Except as noted, at cross-section 2250 located approximately 15-feet upstream of the preferred bridge for the purpose of identifying bridge geometric requirements. 2) Average channel velocity approaching bridge assuming existing abutments removed. 3) At cross-section 2310 located approximately 40-feet upstream of the existing bridge for the purpose of comparison with existing hydraulic conditions. SCOUR AND EROSION Channel Stability: In the vicinity of Blairsden-Graeagle Road, the Middle Fork Feather River channel appears to be substantially in a state of dynamic equilibrium. The channel may however experience transient aggradation events associated with landslides entering Frazier Creek, a tributary entering Middle Fork Feather River a short distance upstream of the bridge. Replacement of the existing Blairsden-Graeagle bridge with the preferred bridge is not expected to affect sediment transport and therefore is not expected to aggravate channel instability. Abutment Local: Abutments of the preferred bridge will not redirect a significant volume of water from the floodplain to the channel during the most probable 100- year flood. Therefore application of the Froehlich Equation in FHWA HEC-18 is precluded. Abutments, however, should be designed considering or protected against potential bank erosion or channel migration. It is not unrealistic to expect up to five feet of additional abutment exposure as a result of bank erosion or channel migration over the expected life of the bridge. 7

11 Pier Local: Contraction Local: Total Scour: Potential pier scour has been estimated to be 13.2-feet using the limiting pier scour equation presented in FHWA HEC-18. The preferred bridge does not constitute a significant contraction of the flood channel. Total potential scour and potential scour elevations at piers are summarized in Table 5. Scour computations and data are included in Appendix C. Location Ground Elev. TABLE 5 Total Potential Scour (feet) Degradation (widening) Contractio n Scour Local Scour Total Scour Scour Elev. Abutment Pier Abutment OTHER CONSIDERATIONS: Drift: Transient Aggradation Existing Bridge: FEMA: There is a large potential for significant volumes of small to medium size drift (branches to small tree trunks) and a modest potential for large drift (large tree trunks) in Middle Fork Feather River. Drift has been considered in the design of the preferred bridge by providing larger span lengths than the existing bridge and more clearance for drift than the minimum recommended. In the event of a landslide entering Frazier Creek, it is likely that sediment will fill the active channel of Middle Fork Feather River to a significant degree during the subsequent transient aggradation event. During such events the water surface elevations of moderate recurrence events can well exceed the water surface elevation estimated for the most probable 100- year flood. While it is impossible to predict or assign a recurrence to these events, they are not particularly uncommon and measures can be implemented to minimize damage should such an event occur. The existing bridge will not be removed after construction of the preferred bridge. The preferred bridge is located within a reach of channel that has flood risk mapped by FEMA using approximate study methods. As such, projects may encroach into the floodplain to the extent they result in a 1.0- foot increase in the water surface elevation of the most probable 100-year flood provided the increase does not result in an increased risk of damage 8

12 to structures or other negative impacts. The preferred bridge is expected to result in a 0.18-foot increase in water surface elevation during the most probable 100-year flood at and for a short distance upstream of the bridge. No structures are in or adjacent to the channel upstream of Blairsden- Graeagle Road therefore the minor increase in water surface elevation during the most probable 100-year flood does not reflect an increase in the risk of damage to structures. No FEMA applications are believed necessary for the bridge replacement project. CONCLUSIONS AND RECOMMENDATIONS Design Flood: Clearance for Drift: Design Exception: Recommendations: Caltrans and FHWA recommend that new and replacement bridges be designed with a minimum soffit elevation equal to the water surface elevation of the most probable 50-year flood (Standard Design Flood) plus appropriate clearance for drift or to the water surface elevation of the most probable 100-year flood (Base Flood) with no clearance for drift, whichever is higher. The minimum clearance for drift recommended by Caltrans and FHWA for bridges over rivers of 3.0-feet is appropriate at this site. None required for flood hydraulic conditions Minimum Soffit Elevation The minimum soffit elevation of a bridge meeting the recommendations of Caltrans and FHWA is ft. This represents the elevation of the Standard Design Flood plus 3.0-feet of clearance for drift. Pier Scour Elevation Pier 2 should be designed considering total potential scour to an elevation of feet. Abutment Scour Elevation Abutment 1 should be designed considering or protected against total potential scour to an elevation of 4339-feet. Abutment 2 should be designed considering or protected against total potential scour to an elevation of 4346-feet. Abutment Protection Recommended to an elevation 3-feet above the water surface elevation during the most probable 100-year flood or to the top of bank, whichever is higher, to reduce risks of damage during a transient aggradation event and of long term potential for damage to abutments from bank erosion and bank migration. 9

13 Note regarding estimates of potential scour: Potential scour has been estimated using empirical equations presented in FHWA HEC-18. These equations do not consider geotechnical conditions and therefore assume all substrate is erodible. The potential scour estimates identified in this report may be inappropriate if a geotechnical investigation identifies material resistant to erosion at higher elevations. Preferred Bridge Characteristics: Soffit Elevation feet (4.59-ft above Q50, 3.13-ft above Q100) Overtopping Flood Impact on Flood Risk Impact on Channel cfs, approximately 150-year recurrence Minor (0.18-foot) increase in water surface elevation during the most probable 100-year flood but no increase in flood risk to structures because no structures are present within the 100-year floodplain. Preferred bridge is not expected to aggravate channel instability. 10

14 Photo 1: Looking downstream (north) at Bridge 09C-0134, Blairsden-Graeagle Road over Middle Fork Feather River Photo 2: Looking upstream (north) at Bridge 09C-0134, Blairsden-Graeagle Road over Middle Fork Feather River 11

15 Photo 3: Looking east across Bridge 09C-0134, Blairsden-Graeagle Road over Middle Fork Feather River Photo 4: Looking west across Bridge 09C-0134, Blairsden-Graeagle Road over Middle Fork Feather River 12

16 Photo 5: Looking downstream at Middle Fork Feather River from Bridge 09C-0134, Blairsden-Graeagle Road Photo 6: Looking upstream at Middle Fork Feather River from Bridge 09C-0134, Blairsden-Graeagle Road 13

17 Figure 1: Sketch of Existing Bridge 14

18 Figure 2: Preferred Bridge Preliminary General Plan 15

19 Figure 3: Middle Fork Feather River Drainage Basin above Blairsden-Graeagle Road 16

20 Figure 4: Site Topography 17

21 Figure 5: Flood Frequency Curve of Middle Fork Feather River at Blairsden Graeagle Road 18

22 Figure 6: Approximate Locations of Surveyed Cross-sections used in Backwater Model 19

23 Figure 7: Existing Bridge as Represented in Backwater Model 20

24 Figure 8: Preferred Bridge as Represented in Backwater Model 21

25 Figure 9: Existing Condition and Preferred Bridge Flood Profiles 22

26 Figure 10: Existing Condition and Preferred Bridge Stage Discharge Curve at Cross-section 2310 for Flood Risk Assessment 23

27 Figure 11: Preferred Bridge Stage Discharge Curve at Cross-section 2250 for Bridge Design 24

28 Figure 12: Existing Condition and Preferred Bridge Stage Discharge Curve at Cross-section 4080 (upstream end of model) for flood risk assessment 25

29 APPENDIX A Additional Hydrologic Figures

30

31 APPENDIX B Additional Hydraulic Data

32 Backwater Model Summary Output, Existing Condition without Drift

33 Backwater Model Summary Output, Existing Condition with Drift

34 Backwater Model Summary Output, Preferred Bridge without Drift

35 Backwater Model Summary Output, Preferred Bridge with Drift

36

37 APPENDIX C Scour Calculations

38

39 APPENDIX D References

40 REFERENCES 1) Caltrans Local Programs Manual. 2) United States Army Corps of Engineers, 1990, HEC-1, Flood Hydrograph Package, Users Manual, Publication No. CPD-1A, Version 4.0, Hydrologic Engineering Center, Davis, CA 3) United States Army Corps of Engineers, 1997, HEC-RAS, River Analysis System, Applications Guide, Publication No. CPD-70, Version 2.0, Hydraulic Engineering Center, Davis, CA. 4) United States Department of Transportation, 1993, HEC-18, Evaluating Scour at Bridges, Second Edition, Report Number FHWA-IP , Federal Highway Administration, Washington DC. 5) United States Department of Transportation, 1991, Hydraulic Engineering Circular Number 20, Stream Stability at Highway Structures, Report Number FHWA-IP , Federal Highway Administration, Washington DC. 6) United States Geological Survey, Harry Barnes Jr., 1967, Roughness Characteristics of Natural Channels, U.S.G.S. Water Supply Paper 1849, U.S.G.S., Denver Federal Center, Denver, CO. 7) United States Geological Survey, Interagency Advisory Committee on Water Resources Data, 1981, Bulletin 17B, Guidelines for Determining Flood Flow Frequency, National Technical Information Service report number PB /AS, U.S. Department of Commerce, NTIS, Springfield, VA. 8) United States Geological Survey, Waananan and Crippen, 1977, Magnitude and Frequency of Floods in California, Report number USGS/WRI 77-21, U.S.G.S., Menlo Park, CA. 9) United States Geological Survey, Water Resources Data for California, Many years, Many volumes, U.S.G.S., Sacramento, CA. 10) Walesh, Stewart G., 1989, Urban Surface Water Management, John Wiley and Sons, New York, NY. 11) Braithwaite, Norman S. Bridge Design Considerations, Non-proximate Geologic Hazards, Floodplain Management Association, 2002, Vol. 3, No. 1, Mission Viejo, Calif. 12) Braithwaite, Norman S. Stream Channel Instability: Cultural Influence and Watershed Planning, Proceedings of the 23 rd Semi-Annual Fall Conference of the California Floodplain Management Association, Integrating Floodplain and Watershed Management, Sept. 3-6, ) MCWA; c:\data\qpro\russian\pwasurvey. 14) Federal Emergency Management Agency, Floodway Flood boundary and floodway map, Mendocino County, CA, (unincorporated areas), Panel 692 of 1100, June 14, 1982, Community- Panel Number B.